Exhaust-gas treatment devices for the purification of the exhaust gases of internal combustion engines in motor vehicles, which devices use a liquid additive for the exhaust-gas purification, are widely used. In exhaust-gas treatment devices, it is for example the case that the exhaust-gas purification method of selective catalytic reduction (SCR) is implemented, where nitrogen oxide compounds in the exhaust gas of an internal combustion engine are reduced with the aid of ammonia.
Ammonia is normally stored in motor vehicles not directly but rather in the form of a liquid ammonia precursor solution. Urea-water solution is particularly commonly used as an ammonia precursor solution. Urea-water solution is available with a urea content of 32.5 percent under the trade name AdBlue® as an operating liquid for exhaust-gas purification in motor vehicles. Urea-water solution may be converted into ammonia within the exhaust gas (within the exhaust-gas treatment device) or outside the exhaust gas (in a separate reactor provided for the purpose outside the exhaust-gas treatment device).
A problem with the provision of operating liquid for exhaust-gas purification in a motor vehicle is that the described operating liquids (in particular the described urea-water solution) may freeze at low temperatures. The urea-water solution with the urea water content of 32.5 percent freezes, for example, at −11° C. Tanks for storing the operating liquid must therefore be constructed such that they are able to withstand the ice pressure that is generated when freezing occurs. Furthermore, it is generally necessary for heating means to be provided by way of which the operating liquid may be melted if the operating liquid has frozen. Furthermore, it is necessary for tanks and delivery devices for delivering the liquid additive to be resistant to urea-water solution.